Well considering at StackExchange their main focus is the StackExchange set of sites, community building Q&A. If you didn't participate then you wouldn't be eat

Well considering at StackExchange their main focus is the StackExchange set of sites, community building Q&A. If you didn't participate then you wouldn't be eating your own dog food and hence would not be as good a developer on the project. Having developers actively work in the community also helps keep them more focused on what they are building, and it means there are more people actually building the ecosystems. Having your own blogs, podcasts, and other public artefacts also helps since it would help build an audience and get the word out to more people about the SE sites. Also don't forget that SE is a startup with VC backing, therefore if the developers don't help in spreading the word the company could fold.